22 THE OCEAN WOULD. 



globe were thus formed of rain water, holding in solution all that the 

 earth had given up, collected in large basins. Chloride of sodium, sul- 

 phates of soda, magnesia, potassium, lime, and silica, in the form of 

 soluble silicate ; in a word, every soluble matter that the primitive globe 

 contained formed part of the mineral contingent of this water. If we 

 reflect that through all time up to the present day none of the general 

 laws of nature have changed if we consider that the soluble substances 

 contained in the water of the primitive seas have remained there, 

 and that the fresh water of the rivers constantly replaces the water 

 which disappears by evaporation we have the true explanation of 

 the saltness of sea water. " It is a very simple theory, it is true," adds 

 M. Figuier, " but one that we have found nowhere, and the responsi- 

 bility of which we therefore claim. The chloride of sodium is by no 

 means the only substance dissolved in sea water. It contains, besides, 

 many other mineral substances : in short, every soluble salt on the 

 face of the globe, and, along with them, portions of different metals in 

 infinitely small quantities." 



The mean temperature of the surface of the sea is nearly the same 

 as the atmosphere, so long as no currents of heat or cold interpose 

 their perturbing influence. In the neighbourhood of the Tropics, it ap- 

 pears that the surface of the water is slightly warmer than the ambient 

 air, but experiments on the temperature of the sea from the surface to 

 the bottom reveal, according to our author,* " some evidence which 

 establishes a curious law. In very deep water a perfectly uniform 

 temperature of four degrees below zero prevails, which corresponds, as 

 physics have established, to the maximum density of water. Under the 

 Equator this temperature exists at the depth of seven thousand feet. In 

 the Polar regions, where water is colder at the surface, this tempera- 

 ture is maintained at four thousand six hundred feet. The isothermal 

 lines of four degrees form a line of demarcation between the Zones, 

 where the surface of the sea is colder, and those where it is warmer 

 than the bed of four degrees below zero." This is more clearly shown 

 in Fig. 4, which represents a section of the ocean, the curved line 

 which touches two points at the surface indicating the depths where 

 the temperature is constantly fixed at four degrees. 



Dr. Maury's account of this phenomenon is asserted with less confi- 

 dence. The existence of an isothermal floor of the ocean, as he calls 



* " La Terre et les Mers," p. 517. Troisieme Ed. 



